It is known that by giving a conjugated diene polymer a radial structure, it is possible to improve various properties compared with a linear conjugated diene polymer. For example, it is known that when used as a rubber material for tire use, by making the conjugated diene polymer a radial structure, the workability and the compatibility with a filler are improved.
Patent Document 1 describes, as a technique for obtaining a radial conjugated diene polymer, to use a lithium amide compound as a polymerization initiator to polymerize a conjugated diene and cause the obtained polymer to react with tin tetrachloride in a coupling reaction to thereby obtain a radial conjugated diene polymer which has amide groups at its ends. According to this method, it is possible to make the conjugated diene polymer a radial structure, then further introduce amide groups at its ends, so it becomes possible to obtain a conjugated diene polymer which is excellent in compatibility with a filler. However, with this method, there is the problem that the functional groups which can be introduced to the ends are limited to amide groups and it is not possible to introduce functional groups with a higher effect of improvement of compatibility with the filler, that is, there is the problem of a low degree of freedom of design of the polymer.
As a technique for making a conjugated diene polymer a radial structure in which functional groups can be freely introduced to the ends, the technique of using a polyvalent polymerization initiator which has a plurality of polymerization start points is known. For example, Patent Document 2 describes a method of polymerization of a conjugated diene using a polyvalent polymerization initiator which is obtained by polymerization of a small amount of divinyl benzene by an organolithium initiator. However, with this method, control of the branched structure is difficult. Further, when using the polymer as a material for tire, there was the problem that the starting end divinyl benzene cross-linked product had a detrimental effect on the performance for tire.
Further, as other techniques using a polyvalent polymerization initiator, the method as described in Patent Document 3 of using a polyvalent initiator obtained by causing an allene compound and organic lithium to react and the method as described in Patent Document 4 of using a polyvalent polymerization initiator obtained by using naphthalene lithium to polymerize a small amount of butadiene, then partially coupling the obtained polymer may be mentioned. However, with these methods as well, there were the problems that control of the branched structure was difficult and, further, the obtained conjugated diene polymer was liable to form a cross-linked structure. Further, with these methods, there was the problem that a considerable amount of linear conjugated diene polymer was unavoidably included and it was difficult to raise the ratio of the radial conjugated diene polymer.
On the other hand, as an example of using a polyvalent polymerization initiator to obtain a polymer other than a conjugated diene polymer, the example described in Non-Patent Document 1 of using a polyvalent lithium compound obtained by an anion transport reaction between a p-methylstyrene oligomer and s-butyl lithium as a polymerization initiator to polymerize the styrene is known. However, there is no known example of application of this method for the production of a conjugated diene polymer.